High-speed continuous flow engines (xe2x80x9cturbomachinesxe2x80x9d) are composed of a series of rotating disks with blades on their periphery. The rotating bladed disks (xe2x80x9cstagesxe2x80x9d) are separated by sets of evenly-spaced stationary vanes, necessary to direct the flow properly into each succeeding stage. Both the stationary vanes and the rotating blades are subject to vibration, usually induced by evenly-spaced circumferential variations of the density, pressure, or momentum of the working fluid of the engine. These vibrations lead to stress that can damage or even fracture the blades.
Blades are subject to resonant vibration at multiple frequencies, each corresponding to a particular vibration pattern or xe2x80x9cmode.xe2x80x9d As an individual blade passes through the field of repetitive variations in the flow field, (xe2x80x9cvane wakesxe2x80x9d), the flow variation exerts a periodic force on the blade much like the force exerted by a picket fence against a stick held by a running child. The periodic force is often called the xe2x80x9cforcing function.xe2x80x9d At particular operating speeds, the forcing function frequency corresponds directly with one of the resonant frequencies of the blade. In that circumstance, the blade vibration amplitude can be destructively high, and cause a fatigue failure of the blade. This sort of blade failure is commonly described as xe2x80x9cHCFxe2x80x9d (High Cycle Fatigue) behavior. HCF failure of a single blade often leads to complete destruction of the engine.
Because of the risks posed by HCF failure, it is important to characterize the vibration modes and fatigue resistance of blades during the engine design and evaluation process. With existing techniques, this characterization is performed on complete engines, and requires expensive and complex instrumentation. Measuring the behavior of an individual stage with these techniques requires many hours of preparation and testing, and the difficulties in acquiring the necessary data makes it impractical to evaluate HCF behavior adequately during engine development. Consequently, HCF failures occur in operation at an unacceptable rate.
One method for controlling the amplitude of the destructive vibration resulting from HCF excitation is to damp the rotor. Engine manufacturers are investigating a number of different ways to achieve a sufficient level of damping without adversely affecting engine performance or lifetime. These new methods all require testing for validation and certification.
Apparatus and methods described herein can be used to test the blades of machinery, including a turbomachine, in an environment that simulates conditions encountered in normal operation, such as centrifugal stress and the high-cycle fatigue (HCF) forcing function. Examples of components for which the apparatus is useful include the compressor, fan, or turbine blades of jet aircraft engines. The apparatus and methods are useful in testing other kinds of machines as well, including, but not limited to, industrial compressors, aircraft propellers, and industrial turbines.
An apparatus for testing vibration in a bladed disk includes a motor-driven rotor upon which a bladed disk can be attached. At least one nozzle coupled with a liquid source is positioned to direct liquid from the liquid source to a position where it would impact at least one blade of the rotating bladed disk. A nozzle can comprise, for instance, a liquid jet for directing a solid stream of liquid onto the bladed disk. In other embodiments, the nozzle can be an atomizing nozzle to distribute liquid over a larger surface area of the blade and increase the time of contact between the liquid and blade. The stress state and position (relative to a datum or other blades) of a rotating blade subjected to the liquid impact can be monitored to determine its vibrational characteristics.
The apparatus generally includes a rotor attached to and driven by a motor or other torque-producing device. A bladed disk can be attached to the rotor and driven as a single-balanced assembly. At least one nozzle coupled with a liquid source is positioned to direct liquid from the liquid source to a position radially extended from the axis of rotation of the rotor such that the liquid can impact at least one blade of the rotating bladed disk. In methods described herein, the stress state and/or position of a rotating blade is monitored as liquid is directed against at least one blade of a rotating bladed disk.
Bladed disk assemblies can be excited at specific frequencies by applying a steady state, controlled periodic force directly to the blades in an evacuated chamber while the disk spins at a fixed speed, or sweeps through a range of speeds. Specifically, the disk and blade rotor assembly or integrally-bladed rotor can be mounted in a vacuum chamber and attached to a drive shaft. A plurality of liquid nozzles can be evenly spaced around the periphery of the disk and arranged so the liquid interacts with the blades as the rotor spins. The blades impart momentum to the liquid, and in doing so are subjected to the reaction impulse. This reaction impulse is the forcing function applied to the blades to achieve the desired vibrational response. The liquid stream/spray can comprise any liquid with vapor pressure equal to or lower than the operating pressure of the chamber.
The apparatus and methods described herein provide a straightforward and reliable means for characterizing the vibration modes and fatigue resistance of blades. One advantage provided by some embodiments of the invention is the ability to control the frequency of excitation by controlling the rate of rotation of the disk and the number of nozzles or nozzle arrays. In addition, the amplitude of the forcing function can be controlled by careful metering of the liquid flowrate. Finally, because a liquid is used to generate the forcing function, the test chamber can be operated at very-low pressure, thereby reducing drag on the rotating blades, and minimizing power requirements.
The apparatus and methods of the present invention permit the application of a variable frequency, adjustable amplitude forcing function to a bladed assembly while the bladed assembly spins at operating speeds. High-speed rotation is important from a blade evaluation standpoint, because the mode shapes, resonant frequencies, and fatigue life of the blades are all strongly influenced by the rotation-induced centrifugal stress encountered during operation.
The invention can be used to characterize and evaluate a variety of rotor variables. For instance, the endurance limit of a part that is simultaneously subjected to centrifugal stresses and HCF excitation can be determined by dwelling at a specified resonance and strain level until the part begins to fail. Also, the damping characteristics of a part can be determined through analysis of strain gage data obtained through an HCF test. Moreover, data from an HCF test can be used to generate forcing functions for use as inputs to analytical models of the object under study. The results of the analysis can then be correlated with test data for model verification.